Out of Spain and so out of the European Union? Could Catalonia secede from Spain, as it is threatening to do, while, as its leaders would like, retaining its membership of the EU?

Its people, after all, are EU citizens now, with all the rights that entails. Can they be deprived of them? But if their EU citizenship arises directly out of their Spanish citizenship, then relinquishing the latter – ironically, even if their government refuses to recognise that act – seems to entail also relinquishing the former.

That would appear to be the position in international law, and back in 2004, as European Commission president, Romano Prodi made it clear, in a statement that caused much grief to Scottish nationalists, that it is the institutional view of the union, one not stated in a treaty, and it was reiterated in recent days by his successor Jean-Claude Juncker.

“When a part of the territory of a member state,” Prodi said, “ceases to be part of that state, eg, because that territory becomes an independent state, the treaties will no longer apply to that territory. In other words, a newly independent region would, by the fact of its independence, become a third country with respect to the union, and the treaties would, from the day of its independence, not apply any more on its territory.”

It would have to apply for membership like any other third-party state – and like any other state face the potential veto of any single member state.

That reality represents the huge political and legal challenge for Catalonia’s secessionist leaders today. The circumstances in which secession happens – specifically, without Spanish acquiescence – will be critical to its ambition to rejoin the union. In truth there is no possibility that an EU application would even be considered by the union should Catalonia decide on a unilateral declaration of independence.

That reality was bluntly explained back in 2011 when Scotland voted in its own independence referendum and the Spanish government, surprisingly, indicated that, in the event of a Yes, it would facilitate the new state’s readmission to the union precisely because the UK government had indicated its willingness to accept the legal referendum result.

But, Spain insisted, it would be a readmission. Spain, like France, the UK, Belgium, Italy and a number of other member states that have seen secessionist movements, has strongly and successfully insisted for years that the EU must uphold the principle that a secessionist entity can never be recognised or rewarded with diplomatic acknowledgment if it does not come into existence by consent.

But what about, some have asked, the European Union’s recognition of Kosovo, which broke from Serbia? The decision was heatedly opposed by several states but in the end agreed on the basis that Kosovo and Serbia were not member states.

What about when the EU suspended its procedures to integrate 17 million East Germans into Germany? Crucially, lawyers argue, no new state was being created

Or what about the willingness of the EU to allow what some see as an analogous expansion of the union, when it suspended its traditional application procedures to integrate 17 million East Germans into the member state of Germany? Crucially, lawyers argue, no new state was being created. And, anyway, the Treaty of Rome was drafted on the assumption that the German federal constitution applied in principle to the entire German territory, but with its application in the eastern states suspended in practice pending reunification.

A similar approach is currently applied to Cyprus in a protocol to its treaty of accession, which recognises that the application of the acquis, or body of rights and obligations binding on all EU member states, is suspended for that part of the island the government does not control.

Mutatis mutandis, the Irish Government hopes, should the day of Irish reunification with a Brexited Northern Ireland occur.

And, whatever the law may say, the EU has a capacity to do remarkable things when there is a common political will.

But the political will in this case is only to demonstrate solidarity with Madrid. The commission has been parroting the same line about the illegality of last weekend’s referendum, and how it’s all an internal affair, for weeks. Police brutality? What police brutality?

There will be no favours for Catalonia, either from the EU or from any of its member states, no matter how sympathetic many of their citizens may feel about Catalan aspirations or their treatment by Spanish police.

A unilateral declaration of independence will be a cold and lonely place. Too much is at stake for it to be otherwise in this union of nation states. No one wants to open the Pandora’s box of secessionism.

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